1. Field of the Invention
The present invention relates to an ophthalmologic apparatus.
2. Description of the Related Art
In a conventional ophthalmologic apparatus such as a fundus camera, a xenon tube or the like is used for a light source for capturing an image of a fundus of a subject's eye. Since the light source generally degrades with time, an amount of the imaging light changes even when the light emission of the light source is normally stopped at the same timing.
Therefore, in order to control the imaging light amount, the light reflected from the subject's eye is monitored by a light receiving element such as a photodiode, and the output of the light receiving element is integrated by an integration circuit equipped with an operational amplifier and a capacitor. Then, there is a known technique of controlling the imaging light amount in a manner such that the light emission is stopped when the output voltage of the integration circuit becomes larger than a reference voltage based on the comparison between the output of the integration circuit and the reference voltage. In addition, there is a known technique of changing the imaging light amount according to the sensitivity of a film to be used for an imaging operation or the interest of an examiner for an imaging region by changing the reference voltage (see Japanese Patent Application Laid-Open No. 60-190930).
However, an available range of the reference voltage is limited by the power supply voltage of the circuit for generating the reference voltage. For example, in a case where the light emission of the light source needs to be stopped when the output of the integration circuit becomes 20 V in the imaging operation requiring a high light amount, if the available range of the reference voltage is 0 to 5 V, the light emission of the light source is stopped when the output of the integration circuit exceeds 5 V and the light amount may not be adjusted. In the above-described example, the power supply voltage of the integration circuit is ignored in order to help the understanding. As described above, there is an issue that the light amount may not be adjusted due to the relation between the reference voltage and the output of the integration circuit.